1. Field of Invention
The present invention relates to an ultra-violet light sensing device, especially an ultra-violet light sensing device with high signal/noise (S/N) ratio.
2. Description of Related Art
Traditional semiconductor based light sensing devices like photo diode are usually used for sensing visible light or infrared light. As for the ultra-violet light, because of its shorter wavelength band (0 nm˜400 nm), the ultra-violet light can only penetrate shallow semiconductor region to induce photoelectrons. However, visible light or infrared light can penetrate deeper semiconductor region to induce photoelectrons, and the induced photoelectrons could drift to the shallow semiconductor region which could interfere the sensing result for the ultra-violet light, because both the drifting photoelectrons and the induced photoelectrons are present in the shallow semiconductor region. In order to improve the S/N ratio in sensing the ultra-violet light, conventional technique utilizes a multilayer filter on the light sensing device, to filter light of undesired wavelength band to prevent it from entering the light sensing device. If the received light contains only the ultra-violet light band, the sensing result is better because there is less interference by the light of longer wavelength band.
However, the yield rate of the multilayer filter manufacturing process is poor. There is a loss in the deposition process for each layer of the filter, and more number of the layers causes more loss in the manufacturing process. Further, the material of the multilayer filter is expensive because precious metal like silver is used, so every manufacturing loss will result in a considerable cost. Therefore, it is desired to improve the S/N ratio of the ultra-violet sensing device, to reduce the manufacturing cost, and to improve the yield rate in the manufacturing process.